1. Field of the Invention
The present invention relates to a photothermal non-destructive evaluation system and method for investigating surface and subsurface features on optical materials. More specifically, the present invention provides a Photothermal Imaging Scanning Microscopy (PISM) system and method capable of investigating full-size optics of up to a meter in diameter in manageable time frames and additionally capable of nanoscale-resolution images by investigating areas of interest.
2. State of Technology
Optical loss in materials plays an important role in high-power laser applications. Losses in optical components often limit the output of high-power laser systems. The losses in dielectric thin films, particularly due to absorption, are usually higher than in bulk optical materials and depend largely on the thin film deposition method used. Therefore, to develop high-quality optical thin films, it is essential that losses be accurately characterized. A number of thermal imaging techniques have presently been applied to characterization of optical materials and in particular to optical coatings. Photothermal Microscopy (PTM), as one such imaging technique, is a tool for non-destructive evaluation of surface and subsurface structures. By measuring localized optical absorption at discrete laser wavelengths, PTM can map optical absorption, scattering and reflectivity, as well as thermal absorption in materials. However, large-area scanning of optical components with such a technique is limited by extremely slow imaging times. For example, imaging a 1 square cm area of a thin film coating having a 10-micron resolution takes about 278 hours. Such a scan time makes PTM non-feasible for full aperture inspection of large-size (e.g., up to about 1 meter) optics.
Background information on a variety of other thermal techniques, including photothermal deformation (e.g., deflection), thermal lensing, and photothermal microscopy (PTM), is contained in “Overview of photothermal characterization of optical thin film coatings,” by Z. L. Wu and M. Thompsen, SPIE, Vol. 2714, (1996), pp. 465–479, including the following: “[o]ne of the most sensitive detection methods for thermal waves is the photothermal deformation technique . . . . This detection technique uses a modulated (periodically modulated or pulsed) laser beam, referred to as the pump beam or heating beam, to generate thermal waves. The detection of the thermal waves is accomplished by using a second laser beam, referred to as the probe beam, which is reflected from the sample surface. The probe beam monitors the slope of the deformation caused by the thermal expansion effect . . . the technique has the ability to characterize local optical and thermal properties, which are of great importance to the study of thin films for high power laser applications.”
Additional background information on PTM and specifically, on using the thermal lensing effect to study thin film coatings is contained in “Non-destructive evaluation of thin film coatings using a laser-induced surface thermal tensing effect,” by Z. L. Wu et. al., Thin Solid Films, 290–291 (1996) 271–277 including the following: “[w]e report a different technique for studying surface deformation by using an optical diffraction effect. In this detection scheme, the laser-induced thermal-bump behaves as a curved mirror which can either focus or defocus a second probe laser bean through optical diffraction, depending on the specific geometry used.”
Accordingly, the present invention provides a quality control non-destructive surface and sub-surface characterization apparatus and method for rapidly scanning large-scale optical components.